1. Field of the Invention
The invention relates to a device for receiving sound waves in wells or drill holes, adapted more particularly for seismic prospection.
2. Description of the Prior Art
The sound wave receiving devices used in wells are generally included in a probe formed from an elongate body having retractable anchorage means associated with drive means.
The anchorage means may be formed from pivoting arms and/or anchorage shoes brought into contact with the wall by means of actuating cylinders associated with hydraulic pressure means controlled from the surface. In a first embodiment, the anchorage means are disposed on the same side of the body of the probe. When they are actuated, the body of the probe is offset laterally and comes into contact with the wall of the well. In the contact zone of the body, in a compartment thereof, are disposed an assembly of sensors. Such a probe is described for example in French Pat. No. 2 501 380.
In another known embodiment, the probe comprises transducers movable with respect to the body of the probe. The transducers are for example rigidly fixed to the rod of hydraulic cylinders and may thus be moved from a retracted position to a coupling position with the wall of the well. Such a probe is described for example in U.S. Pat. No. 3 354 983.
In all cases, the sensors or transducers are fixed or coupled directly to the body of the probe, so that the characteristics of this latter as well as its method of coupling to the earth influence the results of the measurements. The mass of the body of the probe is generally fairly high. It is all the more so since anchorage arms or cylinders and jack means are used proportioned for generating high bearing forces against the wall of the well so as to improve the coupling coefficient of the probe.
Considering the direct couplings generally existing between the sensors and the body of the probe, the mass of this latter influences directly the resonance frequency of said sensors. It can be shown and proved in fact experimentally that the resonance frequency of the sensors increases when the total mass of the body and of the sensors decreases and when the coupling between the probe and the wall of the well is stiffer and also that the frequency response of the sensors improves when the total mass decreases.
The rigid connection of the sensors to the body of the probe also has the drawback that the surface waves which are transmitted by the fluid generally filling the well are detected by said sensors and mask the useful incoming seismic waves.
Furthermore, in the case where the contact between the sensors and the wall of the well is obtained by offcentering the probe, the frequency response of the sensors is modified. It is different depending on whether they are disposed in the plane of symmetry of the offcentered probe or perpendicular to this plane: